Work machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy machinery use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump on the work machine that provides pressurized fluid to chambers within the actuators. An electro-hydraulic valve arrangement is typically fluidly connected between the pump and the actuators to control a flow rate and direction of pressurized-fluid to and from the chambers of the actuators.
During movement of the actuators, it may be possible for gravity acting on the work machine to force fluid from the actuator faster than fluid can fill the actuator. In this situation, a void or vacuum may be created by the expansion of a filling chamber within the actuator (voiding). Voiding can result in undesired and/or unpredictable movement of the work machine and could damage the hydraulic actuator. In addition, during these situations, it may be possible for the actuator to overspeed or move faster than expected or desired.
One method of minimizing voiding and overspeeding is described in U.S. Pat. No. 6,131,391 (the '391 patent) issued to Poorman on Oct. 17, 2000. The '391 patent describes a hydraulic circuit having a tank, a pump, a motor, four independently operable electro-hydraulic metering valves, a motor input pressure sensor, a motor output pressure sensor, and a pump supply pressure sensor. When a pressure measured at the output of the motor is greater than a pressure measured at the input of the motor and the pump supply, an overspeed condition is determined. When an overspeed condition is determined, one of the electro-hydraulic metering valves is actuated to restrict a flow of hydraulic fluid from the motor to slow rotation of the motor and the flow rate of fluid exiting the motor.
Although the hydraulic circuit described in the '391 patent may reduce the likelihood of overspeeding and voiding, it may be slow to respond and may be complex and expensive. In particular, because the mechanism for slowing the motor includes a solenoid-actuated valve, the response time of the hydraulic circuit may be on the order of 5-15 hz. With this configuration, by the time the overspeed condition is determined and counteracted, the effects of voiding or overspeeding may have already been experienced by the work machine. In addition, because the overspeed protection of the '391 patent is based on sensory information, the system may be complex. The additional sensors required to provide the sensory information may also add cost to the system.
The disclosed hydraulic system is directed to overcoming one or more of the problems set forth above.